The present invention relates to a stereophonic radio receiver and more particularly to a receiver wherein a stereophonic reproduction is effectively performed while a sound-to-noise (S/N) ratio is improved.
Due to the straight propagation characteristic of the FM wave, the electric field strength around an FM receiver varies in accordance with a distance from a broadcasting station, geography of the receiving area and dispositions of buildings surrounding the area. In an FM receiver which moves in the varying electric field, such as a car radio, an automatic reception control (ARC) system is provided to change the receiving conditions of the receiver. The ARC system has an automatic separation control system for controlling the separation. Namely, the system controls the degree to which left and right stereo channels are isolated from each other in such a manner that the degree becomes small as the strength of the electric field becomes small. Accordingly, stereophonic sound gradually changes to monophonic sound as the field strength weakens.
A stereo signal is a multiplex signal including frequencies ,in a range between 23 kilohertz and 53 kilohertz arising from a subcarrier. Due to the noises in the above band range, a theoretical S/N ratio is smaller than that of the monophonic broadcast. Namely, as shown in FIG. 2, the S/N ratio of an FM tuner at the stereophonic sound reception is theoretically decreased by 21.7 decibels from that of the monophonic sound reception.
FIG. 3 shows a conventional FM receiver having a separation controller intended to improve the S/N ratio in consideration to the above-described problem.
A stereo broadcast wave is tuned in and converted into an intermediate frequency signal at a front-end 1. The intermediate frequency signal is amplified at an intermediate frequency amplifier 2 and demodulated at an FM detector 3. A stereo decoder 4 demodulates a sum signal (L+R) on a main carrier and a difference signal (L-R) on a subcarrier. The sum signal and the difference signal are added to each other, thereby reproducing right and left channel signals.
An amplified intermediate frequency signal is applied from the amplifier 2 to a level detector 5 where the electric field strength represented by the transmitted stereo signal is detected. The detected electric field strength is applied to a separation controller 6 which produces a separation control signal in accordance with the electric field strength. The separation control signal is applied to the stereo decoder 4 to control the separation of the right and left channel signals.
More particularly, when the electric field strength decreases, a demodulating rate of the subcarrier, which includes noises, is decreased, thereby reducing the difference signal (L-R). When the difference signal is added to the sum signal (L+R) to produce the right and left channel signals, the noise is restrained. Thus, the S/N ratio is improved as an exponential function of a demodulating rate of the subcarrier as shown in FIG. 3.
Hence, in a strong electric field, the modulation rate of the subcarrier is increased, thereby clearly separating the right and left signals. In a weak electric field, the demodulation rate of the subcarrier is reduced, thereby improving the S/N ratio.
However, in the conventional FM receiver, the separation control is not dependent on the level of the signal which decides the S/N ratio, but dependent on the strength of the electric field. Therefore, the separation is unnecessarily reduced in spite of high level of signal. Namely, the stereophonic sound is often changed to the monophonic sound. Therefore, the period of the stereophonic reception is reduced.